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August 2010

August 13, 2010

I don't mean to recycle a tired, old topic here, but I was recently asked by a newspaper columnist for an opinion on the relative impacts of paper, plastic and cloth shopping bags. This is interesting once again because of the plastic bag ban in San Francisco, to be followed this year by my hometown Portland. The ban is now common in parts of Europe and Asia, including Bombay and Bangladesh. The litter and the difficulty in recycling these bags are real problems, so I am personally glad to get rid of this nuisance as long as there is a viable replacement.

A quick carbon footprint analysis shows that the reusable plastic "green bag" is a good option: it takes about 18 uses to break even with single-use HDPE bags and just about 4 uses to break even with single-use paper bags, and additional uses after that are basically emissions-free. On the other hand, the light-weight cotton bag takes over a 100 uses before it breaks even with HDPE. This assumes that the cotton bag is washed rarely, the paper bag is recycled after use, and all plastic/cotton bags are landfilled at end of life.

August 12, 2010

Residential lawns are a fixture in every US suburb. I periodically hear about the resources that go into maintaining the 20 million acres of these lawns and it came up again in a recent discussion of GHG reduction strategies for Oregon, so I decided to check it out.

An estimate from the EPA suggests that 30-60% of urban fresh water is used for watering lawns, and the US Geological Survey provides good estimates for public-supply water in the US. Lawn mowers consume about 580 million gallons of gasoline annually. The Pesticide Action Network estimates that 90 million pounds of herbicides are dumped on our lawns each year. Lawn fertilizer use varies from 2-4 lb of nitrogen annually per 1000 sq ft of lawn for most grasses.

Taking the low end of the various estimates, I estimate that the annual GHG emissions from residential lawns is less than 0.25% of the total US GHG emissions. For a typical home with a 1000 sq ft of lawn, the annual GHG footprint is just over 18 Kg CO2e, about the same as a kg of typical beef.

(I have made some assumptions including near-complete combustion of the gasoline used in lawn mowers, which may not be quite accurate, but I am only aiming for a ball park answer here.)

Here is a breakdown of the emission sources:

The climate change impact is minuscule compared to many other household emissions. Other pollution (both air and noise) might be more of a concern. What is really alarming is the water footprint: over 20,000 L of fresh water is needed to irrigate a typical 1000 sq ft lawn each year -- that is some 3500 flushes of the toilet!